Overnight index swaps are interest rate swaps where an agreed index of an overnight floating rate, such as SONIA or EONIA or the Fed funds rate, is exchanged for a fixed rate for an agreed period. They are often used by banks to hedge against fluctuations in official interest rates set by central banks.

See also: http://www.acisuisse.ch/docs/dokumente/OIS_Note_CSFB_Zurich.pdf, LIBOR-OIS Spread